The wild-type (WT) RUNX1 genomic locus produces messenger RNA (mRNA) transcripts from either the P1 or P2 promoter generating RUNX1c or RUNX1b protein isoform, respectively. RUNX1c is expressed in preMegE progenitor cells, MkPs, and early erythroid progenitor cells, whereas RUNX1b is expressed in PreMegE cells as they commit to MkPs and during Mk maturation. Draper et al engineered the P1-MRIPV allele by replacing DNA sequences in exon2 that encode the normal MASDSIFESFPSYPQCFMR amino acid sequence (unique to RUNX1c) with those encoding MRIPV (unique to RUNX1b). Homozygous P1-MRIPV knock-in mice, which lack expression of all RUNX1c, have impaired survival of MkPs, reduced megakaryocyte output, and enhanced erythroid output. The MkPs that survive have normal terminal megakaryocyte maturation. The figure has been adapted from Figures 1 and 7 in the article by Draper et al that begins on page 271.